I wouldn't want to offend Mozart by trying to describe his hometown in two sentences.
Otherwise Salzburg is a really beautiful and aristocratic town. Its name means Salt Fortress; you will find out why when you climb the hill with the old castle. The people are nice, the food is tasty, the music is classical.
I would advise you to set Salzburg as your first priority if you go to Austria instead of the overpraised Vienna.
The best time of the year to visit this town is by all means the end of january during the annual Mozart Week. It is the time of the big concerts, too.